A cogeneration system using a fuel cell typically includes a hydrogen generator having a reformer, and a fuel cell. The reformer includes a reactor filled with a reforming catalyst for catalyzing a steam reforming reaction, and a combustor for heating the reactor with combustion heat. A material gas such as a hydrocarbon based gas and a steam are flowed in the reactor which has been heated, causing the steam reforming reaction to occur. As a result, a hydrogen-containing gas is generated. The hydrogen-containing gas which has been exhausted from the reformer is returned to the combustor through the fuel cell or the like and is combusted therein, thereby heating the reactor. The heating enables the supply of the heat necessary for the steam reforming reaction which is an endothermic reaction.
In some cases, the hydrogen generator includes a shift converter and a purifier to reduce a carbon monoxide concentration in a hydrogen-containing gas. In such a configuration, the shift converter and the purifier are required to be maintained at temperatures suitable for the reaction. The shift converter and the purifier are caused to rise in temperature by flowing a high-temperature hydrogen-containing gas therein. In addition, by supplying air to the shift converter and the purifier, the hydrogen-containing gas reacts with the air to generate reaction heat, which makes it possible to heat the shift converter and the purifier efficiently.
The air is supplied to the combustor to maintain combustion therein. If the combustor is deficient in air, then imperfect combustion occurs, causing flame extinguishment or generation of carbon monoxide. In order to prevent occurrence of the imperfect combustion, in a combustor disclosed in a patent document 1, the amount of air supplied to the combustor is determined based on a supply amount of the material gas so that a concentration of oxygen remaining in a gas exhausted from the combustor is maintained at a certain value or higher.
Patent document 2 and patent document 3 disclose a method of controlling the flow rate of air supplied to a burner provided in a reformer of a fuel cell. In the method disclosed in the patent document 2, the amount of air supplied to the burner is controlled based on the flow rate of a source fuel. In the method disclosed in the patent document 3, the amount of air supplied to the burner is controlled based on a power generation electric current or a response time associated with reforming a raw material. The patent document 3 further discloses, in a prior art description, a method in which an oxygen concentration in a combustion exhaust gas is analyzed and the air is supplied to the burner such that remaining oxygen concentration is 2 to 3% or higher.    Patent document 1: Japanese Laid-Open Patent Application Publication No. 2002-267159    Patent document 2: U.S. Pat. No. 3,212,181    Patent document 3: U.S. Pat. No. 3,718,391